Logs on Nerdsid.com

Practical Grep: Filtering Errors and Patterns in Log Files

Tue, 30 Jun 2026 11:41:54 +0000

Introduction

This walkthrough covers using grep to search and filter log files for errors, specific requests, and multi-pattern matches.

Tasks

Find all ERROR messages in system.log and count how many there are.
Find all lines containing POST requests in access.log.
Find all lines in application.log that contain both WARNING and query (case-insensitive).
Search for all user authentications (containing User authenticated) across all log files.

Requirements

Use grep for all searches (combining with other commands like wc where needed).
Save each task’s output to its own file: task1_output.txt, task2_output.txt, task3_output.txt, task4_output.txt.
Original log files must not be modified.

Solution

Task 1

 
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grep -c "ERROR" system.log > task1_output.txt

grep -c counts the number of matching lines instead of printing them. Redirecting the output saves the count to task1_output.txt.

Linux Journalctl Command Cheat Sheet

Wed, 15 Apr 2026 06:39:17 +0000

I recently needed to free up disk space by removing old system logs, so I collected the command I used and other useful journalctl commands for future reference.

Disk usage & vacuum (remove old logs)

Show journal disk usage:

 
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sudo journalctl --disk-usage

Remove archived journal files until total size is below X:

 
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sudo journalctl --vacuum-size=500M

Remove archived journal files older than a time:

 
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sudo journalctl --vacuum-time=3d

Remove archived journal files, keeping only the most recent N files:

4 Ways to Watch Logs Live in Linux

Wed, 15 Oct 2025 12:49:55 +0530

1. `tail -f`

Use when:

You want to quickly stream a log file.
You want to perform short, one-off checks on a single log file.

Availibility: available on all Linux/Unix systems.

Example:

 
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tail -f /var/log/syslog

2. `tail -F`

Some services (like Nginx, Apache) rotate logs i.e. they stop writing logs to an old file and instead write to a new file (named sequentially). tail -f stops following when that happens, but tail -F automatically switches to the new file. In other words, it handles log rotation gracefully.